Positron emission tomography (PET) is used to generate high resolution images of organs and tissues of the human body. These images aid in medical treatment for a patient, and are prepared by infusing the patient with a radiopharmaceutical substance that includes a short-lived radioisotope, and then scanning the patient for gamma radiation. Briefly, the radioisotope undergoes radioactive decay as the radiopharmaceutical substance distributes within the patient's body. The radioactive decay results in positron emission. The emitted positrons are annihilated, which results in the production of gamma photons that are detected using an array of gamma detectors that encircle the patient. After a can is complete, an image is constructed by a computer using tomographic algorithms. The radiopharmaceutical substance must be used before the radioisotope undergoes so much radioactive decay that it becomes useless for the production of images.
Generators of radioisotopes for radiopharmaceutical substances are a cost effective alternative to on-site production, which typically requires a cyclotron (which is an expensive and complex apparatus), staff for operating the cyclotron and handling the radioactive targets, and a laboratory for processing the irradiated target into a generator. The most desirable generators of radioisotopes for radiopharmaceutical substances use a relatively long-lived parent radioisotope that continually produces, by its radioactive decay, a short-lived daughter radioisotope with desirable imaging or therapeutic properties. In equilibrium, both parent and daughter have the same activity, which decreases over time according to the half-life of the longer lived parent. This allows time for processing and shipment of a generator from a production site while supplying a continuing source of the daughter nuclide for use at a nuclear medicine clinic.
Radioarsenic is a useful radioisotope for radiopharmaceuticals. Radioarsenic offers four positron-emitting radioisotopes (70As, 71As, 72As, and 74As) and three beta-emitting radioisotopes (74As, 76As, and 77As). Thus, radioarsenic-based radiopharmaceuticals may be used as imaging agents as well as therapeutic agents. The half lives (t1/2) of these radioisotopes range from 53 minutes to 18 days. Positron emitting radioisotopes may be used for positron emission tomography (PET) imaging, while beta emitting radioisotopes may be used in internal radiation therapy where radioactivity is brought close to a tumor.
72As is a positron emitter with an emission rate of 88% and an energy E(β+max) of 2.5 MeV. 72As has a half life of 26 hours, which is short enough to limit the radiation dose delivered to the patient while long enough to allow for chemical modification prior to targeted delivery. Thus, 72As is suitable for PET imaging.
72As is formed via the decay of 72Se (t1/2=8.5 days, EC 100%). The 72Se/72As parent/daughter generator is similar to the 82Sr/82Rb system currently used in many nuclear medicine procedures in the United States. 72Se has been produced as a component in a mixture of radioisotopes by charged particle bombardment of a target. Common targets include arsenic, germanium, or bromine.
A variety of approaches for preparing 72Se/72As generators have been reported. One approach involves repeated distillation of 72AsCl3 from carrier added 72Se stock solutions. Another approach involves electroplating 72Se as Cu2Se on Cu backings. Yet another approach involves solid phase extraction of 72Se. The first two approaches require elaborate radiochemical manipulations that render them unsuitable for rapid on-site handling by nuclear medical technicians. The third appears to be limited to very small dimensions, requires the use of high concentrations of bio-incompatible agents such as hydrofluoric acid, and uses elemental Se which requires an inert atmosphere to prevent oxidation, which may be difficult in a clinical setting.
Better methods for preparing 72 Se/72 As generators remain desirable.